Oscillation generator control circuit



Sept'. 20,l 1949. L, A. KING OSCILLATION GENERATOR CONTROL QIRCUIT Filed 001,. 5, 1945 ATTORNEY Patented Sept. 2l), 1949 OSCILLATION GENERATOR CON'OI'RO amount Louis A. King, Haddoniield, N. J., assigner to Radio Corporation of America, a corporation of Delaware Application october 3, 1945. Seial. No.v 620,151.,

3 Claims. 1i This invention relates to a high frequency heating system, and particularly to the power generator circuit.

The present invention provides an electron discharge ydevice induction heating system having among its features,v a motor-controlled variable grid resistor for controlling the output power over a limited range, a bias rectiiier arrangement including a relay for opening the grid circuit to stop oscillation and immediately apply-ing a xed vholding bias to the grid to prevent intermittent oscillation, and a protective discharge gap to protect the Variable grid resistor and bias supply from excessive grid circuit voltage due to 'possible iiashl-arcs in the oscillator tube.' n y 5 4A more detailed description ottheinvention follows in conjunction with a drawing 'whose single gure schematically illustrates the essential circuit elements ofthe power generator of the invention used'in aninduction heating sysfr tem.

Referring to the drawing, there is shown an induction heating system comprising a vacuum tube III having its electrodes coupledy to an oscillatory circuit I2 to form a Hartley oscillator. The anode ofthe tube is connected via blocking condenser II( to one terminal ofj the oscillatory circuit, while the grid is connected to the other terminal via blocking condensers I3. The concentric-cables 9, 9 connecting the oscillatory circuit to the vacuum tube Il) are a radio frequency high voltage type and enable the applicator unit to be located at some distance (,50 it. for example). from the tube III.

The oscillatory circuit comprises the primary winding of anY 'oil-filled ra'dio frequency current transformer T andrl a gas filled xed capacitor I5 connected in shunt tothe primary winding. Several turns on the primary'winding'are connected to different terminals of a rotary tap switch I4 whose rotatable arm is connected to. ground. This switch serves as a feedback or` excitation control for the oscillator.

The secondary winding ci rtransformer T is a single turn which is directly connected to an appropriate applicator circuit, for` example, a single high current low impedance loop 4i! shapedV to follow the contour of the metallic object Lto be heated.

The anode of the tube I II is supplied with posiu tive polarizing potential from a source +B via an inductance coil I6. The filament of the tube I0 is supplied with suitable heating current from an alternating current supply over transformers I1, I8 and I9 having `their primary windings eonenable f pins oscillations- 2 nected in parallel relation and their secondary windings also connected in parallel relation. These three filament transformers have been ern-- ployed for weight considerations, and it should be understood that, ij desired, they can be replaced by a single transformer. Reactors 2D, 2I and 21, are provided in series with the primary windings of these filament transformers in order 'tol limit the iilament starting current to 15.0% of normal running ourrentt 1 In circuit with the grid oftube Il) is an inductor 23 in series with a resistor 24 and avpotentiometer 25. The tap on potentiometer 25 is linked to the shaft of a motor Mw-hich is controlled from a switch (not shown) on aI panel located at the applicator unit which is somewhat removed from the tube IQ vand associated elements. This rheostat 25 enables the operatorr to control the power output over a limited range,let us say 10%. An indication of the power output is given on the output meter I in circuit with the secondary winding of transformer T. Thus, if the rated power output of the generator is '75 kw., an adjustment of the, rheostat 25 which will introduce its greatest amount of resistance may reduce vthe power output to, let us say 68 kw. Fur-ther reduction in power output may be obtained by reducing the anode voltage byV means of reduced voltage taps on the main rectifier anode transformer (not shown).

In order to supply bias to the tube I0 for diferent operating conditions, there iS provided in circuit with thegrid a biasrectiler arrangement which includes a voltage step-up transformer'EG', a resistor 21, and a dry disc rectier 28. The contacts I and 2.0i relay 29 are enclosed in an 'evacuated envelope 3l). The armature within envelopefSO is magentically operated by the relay winding. This relay 29 is connected to a suitable control circuit via leads 3l for starting or stop- V This control circuit may cornprise a suitable interval timer and/or start and stoprnanually operated switches, or limit switches associated with the travel of the work.

When the armature or .relay 29 engages 00n- .tact 2 (normal position) there is supplied :to the a frlroiit (tia resistor 21'). a bias supply which prevents the occurrence of intermittent voscillation (motorboating) of tube lf3 due to the large R.C. time constant of the grid circuit when in this position since resistor 2l is large in resistive value. When the armature engages contact l, the fixed bias supply is disconnected -from the grid, and the grid circuit is set up to the production of oscilla-tions by tube Il) by virtue of a grid return circuit which includes lead 4 I, ammeter A, winding of overcurrent relay 32, lead 33 and the center tap of lament transformer I1. When the armature moves again to engage contact 2, the grid current is interrupted due to a large self bias produced by the increasingly high resistance between the relay contacts during the parting stroke from contact I t-o contact 2. If some xed bias be not then applied, motorboating will occur at a rate determined by the effective R.C. time constant of the grid circuit.

Past experience indicates that high vacuum high voltage power tubes are subject to flash arcs which usually occur between anode and grid or cathode. These high resistance arc discharges, if of suiiicient energy. can cause sufcient ionization for a low resistance power arc to follow. In the event of such an occurrence between grid and anode, the grid will go to anode potential thus subjecting all elements in the grid circuit to high voltages which will puncture the normal insulation of these elements. In view of the possibility of such an occurrence, the protective gap 34 is employed to effectively ground all elements of the grid circuit from the junction of resistor 24 and rheostat 25 to ground if the grid circuit voltage exceeds a prescribed safe value. This gas filled protective gap may, for example, be designed to breakdown at 1200 volts or 2200 volts, depending upon the insulation level ,of the grid circuit.

The invention was designed particularly for use with a 400 kc. Hartley oscillator giving a power output of 75 kilowatts. In this design, tube l was an RCA type 9G21 water cooled device to whose anode should be supplied 17,000 volts direct current (positive). The resistor 24 in the grid circuit was 1300 ohms, while the rheostat had a range from zero to 2000 ohms, depending upon its variable position. Obviously, the features of the invention are not limited to these details since different circuit elements requiring different constants and voltage values can be used, depending upon the results to be achieved.

In order not to detract from the essential details of the invention, no illustration has been made of the water cooling equipment for the oscillator tube which includes a iiuid circulating pump, heat exchanger, iiuid storage tank, ceramic insulating coils, and other elements mounted in the oscillator tube unit, although it should be understood that this apparatus is customarily employed with high frequencyY heating systems.

The term "ground used in the appended claims is deemed to include any point or surface of zero potential for D. C. or alternating current.

What is claimed is:

l. A high frequency heating system comprising an electronic power generator having a high vacuum high voltage power triode tube subject to flash-over, said tube including an anode and a control grid, an oscillatory circuit for said generator coupled between said anode and control grid, the control grid for said tube including a iixed resistor, means for varying the power output from said system over a limited range, said means including a potentiometer in series with said ixed resistor, said potentiometer serving to control the power output of said generator over a limited range, a motor for operating said potentiometer, and a switch remotely located with respect to said motor for controlling said motor, and a protective circuit comprising a gap con` nected between ground and the junction point of said xed resistor and potentiometer, said gap functioning to ground all elements of the grid circuit from said iunction point to ground in the event of excessive grid circuit voltage.

2. A high frequency heating system comprising an electronic power generator having a high vacuum high voltage power tube subiect to iiashover, said tube including a cathode, an anode and a control grid, an oscillatory circuit for said generator coupled between said anode and control grid, a xed impedance and a potentiometer in series with said grid, a protective gap connected between ground and the junction point of said xed impedance and potentiometer, a relay having an armature magnetically controlled by a winding and a pair of contacts for alternately engaging said armature, said armature and contacts being enclosed in an evacuated envelope, a D. C. connection from said grid to said armature, a connection from one of said contacts to said cathode to enable the production of oscillations by said generator, and a'source of fixed bias coupled to said other contact, said source including a resistor shunted by the series circuit of a rectiiier and the secondary winding of a step-up voltage transformer, the value of said bias being such as to prevent oscillations by said generator when said armature engages said last contact.

3. A high frequency heating system comprising an electronic power generator having a high vacuum high voltage power triode tube subject to flash over, said tube having a control grid circuit including a xed resistor, means for varying the power output from said system over a limited range, said means including a potentiometer in series with said iixed resistor, said potentiometer serving to control the power output of said generator over a limited range, a motor for -operating said potentiometer, and a protective circuit comprising a gap connected between ground and the junction point of said xed resistor and potentiometer, said gap functioning to ground all elements of the grid circuit from said junction point to ground in the event of excessive grid circuit voltage.

LOUIS A. KING.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,198,700 Colpitts i Sept. 19, 1916 1,728,816 White Sept. 17, 1929 1,919,993 Tripp July 25, 1933 2,262,149 Slonczewski Nov. 11, 194] 

